Solid oxide fuel cells (SOFCs) are used to generate electrical energy through electrochemical reactions between air and hydrocarbon fuel gas to produce a flow of electrons in an external circuit. Generators based on SOFCs offer a clean and highly efficient approach for electrochemical generation of electricity. Conventional SOFCs typically include a ceramic air electrode or cathode in the form of a porous tube made of doped lanthanum manganite. The outer surface of the air electrode is mostly covered by a solid electrolyte material made of yttria-stabilized zirconia. The outer surface of the solid electrolyte is typically covered with a cermet fuel electrode or anode made of nickel-zirconia.
Conventional solid oxide fuel cells are disclosed in U.S. Pat. Nos. 4,395,469 to Isenberg, 4,476,196 to Poppel et al., 4,476,198 to Ackerman, et al., 4,490,444 to Isenberg, 4,562,124 to Ruka, 4,751,152 to Zymboly, 4,767,518 to Maskalick, 4,888,254 to Reichner, 5,106,706 to Singh, et al., 5,108,850 to Carlson, et al., 5,277,995 to Ruka, et al. and 5,342,704 to Vasilow, et al. Each of these patents is incorporated herein by reference.
The air electrodes of conventional solid oxide fuel cells typically have porosities of from about 20 to 40 percent, and also have good electrical conductivities. The air electrodes are usually comprised of oxides having a perovskite-like crystal structure (ABO.sub.3), such as LaMnO.sub.3 wherein the La occupies the A-site and the Mn occupies the B-site. Conventional SOFC air electrode materials based on LaMnO.sub.3 are typically made with high purity lanthanum oxide, which is costly. In order to reduce raw material costs, the use of lower cost mixed lanthanides as replacements for high purity lanthanum in the production of air electrodes has been investigated.
An article entitled "Ln.sub.1-x AE.sub.x MnO.sub.3 (Ln=Lanthanum Concentrate Lanthanoids, AE=Sr, Ca) as a Cathode Material in Solid Oxide Fuel Cells", by Mori et al. of the Central Research Institute of Electric Power Industry-Japan, discloses air electrode compositions synthesized using a mixed lanthanide. The compositions proposed by Mori et al. are of the general formula Ln.sub.1-x AE.sub.x MnO.sub.3, where Ln is a mixed lanthanide and AE is Sr or Ca. Unlike typical mixed lanthanides which contain La, Ce, Pr and Nd, the mixed lanthanide disclosed by Mori et al. is a left-over rare-earth mineral after Nd separation. This material comprises mainly La, Ce, Pr and a trace amount of Nd. Mori et al. disclose that by varying the A-site doping level (either Sr or Ca) in the air electrode composition, adequate coefficient of thermal expansion (CTE) and electrical conductivity properties can be achieved. However, there are several drawbacks associated with this approach. The issue of dimensional stability is not addressed. The CTE values are controlled by the Sr or Ca doping level. In order to achieve a desired CTE value of, for example, 10.5.+-.0.1.times.10.sup.-6 /.degree.C., the Sr doping has to be close to 15 atomic percent and the Ca doping must be close to 30 atomic percent in the A-sites. This requirement significantly affects the flexibility for composition adjustments needed to optimize other critical air electrode properties. Although Mori et al. does not disclose thermal expansion properties, it is mentioned that both of the Sr-doped and Ca-doped compositions exhibit anomalies in the thermal expansion curves between room temperature and 1,000.degree. C. Such anomalies are typically due to a phase transition (rhombohedral to orthorhombic transformation for the Sr-doped compositions and hexagonal to tetragonal transformation for the Ca-doped compositions) which can occur in this temperature range. Such anomalies can be minimized by proper doping in the A-site. The compositions disclosed by Mori et al., however, do not have the flexibility for such a composition adjustment since either the Sr or Ca doping level needs to be fixed at a given level in order to achieve the desired CTE value.
U.S. Pat. No. 5,686,198 to Kuo et al. filed Feb. 29, 1996, which is incorporated herein by reference, discloses the use of mixed lanthanides in making SOFC air electrodes. Two general formulas for air electrode compositions are disclosed: (La.sub.1-w-x-y- Ln.sub.w Ce.sub.x Ma.sub.y)(Mn.sub.1-z Mb.sub.z)O.sub.3 ; and (La.sub.1-w-0.2 Ln.sub.w Ca.sub.0.2)(Mn.sub.1-z (Ni or Mg).sub.2)O.sub.3. In both formulas, Ln is a mixed lanthanide comprising La, Ce, Pr, Nd and Sm, Ma is Ca, Sr or Ba, and Mb is Mg, Ni, Cr, Al or Fe. In these compositions, the CTE values are primarily controlled by the La/Ln ratio.